The invention relates to a scanner for the optical scanning of objects, in particular recording discs, comprising a coherent light source; a light deflecting device which allows a light beam emerging from the light source to sweep over the object in a substantially linear scanning movement, with the optical axis of the light beam subtending a small angle with a normal to the object surface transverse to the scanning direction; and a detector for detecting light which is reflected back from the object.
Such scanners are used in the cheking of CD recording discs (compact discs) which accompanies the production thereof. Compact discs are metallized plastic discs coated with a transparent protective layer with a digital information, in particular an audio signal or a video signal being accommodated on the metallized plastic discs in the form of microscopic elevations and depressions. For the replay the CD recording discs are read out with a laser beam which is reflected at the metallic layer. A scanner used in the inspection of CD recording discs operates in accordance with the same principle. This makes it possible to scan the disc surface without gaps using a rapidly deflected laser beam with simultaneous rotation of the disc, and thereby to detect disc faults which produce a change of the reflected and/or diffracted laser light.
It is known to allow the coherent laser light to be incident on the CD recording disc perpendicular to the scanning direction as seen in the scanning direction parallel to the normal to the disc, but at a small angle to the normal onto the disc. The angle serves in this arrangement for the separation of the incident light and the light reflected back by the disc. This angle is small, for example about 1.5.degree., so that no notable enlargement of the light bead occurs above the reflective metal surface and transverse to the scanning direction through the separation of the incident and reflected light, and so that a correspondingly high measurement accuracy and fault sensitivity is also achieved for faults above the metal surface--in the substrate or at the read-out side.
The incident laser beam penetrates the about 1.2 mm thick plastic layer of the CD recording disc and is reflected from the metallic reflective coating lying beneath it. During this the information pattern of the CD disc, which acts as diffraction grating, generates several discrete diffraction orders. The reflected light of the zero diffraction order and of the discrete higher diffraction orders is multiply reflected at the boundary surfaces between air and plastic on the one hand and plastic and plastic at the reflective coating on the other hand, with each laser beam generating discrete diffraction orders anew on reimpingement on the information tracks of the CD recording disc. In this manner, with corresponding geometry of the boundary surfaces relative to one another, a superposition of light beams causes interference fringes of constant thickness. This leads to significant modulation of the received signal generated by the reflected and/or diffracted light coming from the disc. This in turn makes the detection of manufacturing faults in the CD recording disc more difficult and reduces the accuracy of the testing method.